xdsclient: stop caching xdsChannels for potential reuse, after all references are released #13189

Job	Run time
vet-proto	1m 2s
vet-proto	1m 2s
tests (vet, 1.22)	3m 3s
tests (vet, 1.22)	3m 3s
tests (extras, 1.23)	3m 39s
tests (extras, 1.23)	3m 39s
tests (tests, 1.23)	4m 22s
tests (tests, 1.23)	4m 22s
tests (tests, 1.23, -race)	5m 34s
tests (tests, 1.23, -race)	5m 34s
tests (tests, 1.23, 386)	5m 10s
tests (tests, 1.23, 386)	5m 10s
tests (tests, 1.23, arm64)	6m 32s
tests (tests, 1.23, arm64)	6m 32s
tests (tests, 1.22)	4m 57s
tests (tests, 1.22)	4m 57s
tests (tests, 1.23, -race, GRPC_EXPERIMENTAL_ENABLE_NEW_PICK_FIRST=true)	5m 0s
tests (tests, 1.23, -race, GRPC_EXPERIMENTAL_ENABLE_NEW_PICK_FIRST=true)	4m 52s
	1h 18m 30s

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